1. Field of the Invention
This invention relates to bicycle framesets and more particularly to lightweight high performance bicycle framesets expected to be subjected to severe impact loads encountered in off-road racing.
2. Prior Art
Racing bicycle framesets used on dirt tracks and for off road "mountain" racing require a combination of construction features which include high strength, stiffness, and low weight. Frame rigidity and bicycle weight are two interrelated factors which are very significant, particularly for high performance racing bicycles. The lower the bicycle weight, the greater the pedalling responsiveness of the bicycle. Similarly, the more rigid the frame, the greater the pedalling ease. Ideally, bicycle frames should exhibit minimal torsional and transversal flexing caused by pedalling effort and steering maneuvers. The classic diamond shaped bicycle frame is subject to bending and torsional stresses, particularly when the rider exerts pressure first on one pedal and then the second pedal. Flexing in the bicycle frame uses up rider energy which might otherwise power the bicycle in a forward direction. Flexing can also destabilize bicycle steering in a high speed downhill race. Bicycle frames should ideally be practical to manufacture and be so constructed that a minimal amount of material is used to manage the stresses from the various loads in order to minimize weight.
Prior art inventions have fulfilled some of these requirements in one or more of the following ways: (1) by making special round tubular frame elements in which the wall thickness of the tube increases where stress is concentrated; (2) by employing improved manufacturing processes, particularly welding and heat treating processes for joining lightweight alloys such as titanium and aluminum; (3) by increasing the diameter of certain elements which are assumed to carry greater loads; (4) by employing stiff, lightweight composite materials, the shape of which can be varied more practically than with metallic tubular elements, such as U.S. Pat. No. 4,900,048; and (5) by wrapping a metallic tubular frame with a composite material of variable thickness or with composite fibers oriented to maximize structural efficiency, such as in U.S. Pat. No. 4,047,731.
Prior art bicycle framesets typically fulfill one requirement at the expense of another, or require impractical means of manufacture. In some prior art inventions, such as U.S. Pat. No. 2,493,037, all of the walls of the tubular elements do not connect to the walls of other critical elements, creating stress concentrations and reductions in strength, stiffness and fatigue life, because loads are not properly transferred among the various elements. Other prior art inventions that employ combinations of composite and metallic materials in their construction are difficult to manufacture or tend to have low joint efficiency where members are bonded together. Certain other prior art inventions achieve considerable strength and stiffness vertically and longitudinally to the direction of travel, whereas transversal and torsional strength and stiffness do not reach high values.